Several purified rabbit muscle and liver enzymes were administered to goats as antigens. High affinity antibody preparations have been obtained against muscle phosphofructokinase, fructose diphosphatase and pyruvate kinase and against liver fructose diphosphatase. It is planned to raise antibodies also to liver phosphofructokinase and pyruvate kinase. Thus, the enzymes in most cases have complex quaternary and play important regulatory roles in glucose utilization and formation. The specific antibody preparations are evaluated by gel immunodiffusion methods and by their ability to inhibit assayable activities of the purified enzymes and of the enzymes present in tissue homogenate preparations. It is proposed to elaborate on suitable iodination methods for muscle phosphofructokinase and pyruvate kinase and for liver fructose diphosphatase. The iodination will be done in a way that preserves quaternary structure, catalytic activity and immunologic reactivity. When we purify liver phosphofructokinase and pyruvate kinase, we will raise antibodies and define suitable iodination conditions for them also. Subsequently, radioimmunoassays for the native enzymes will be developed providing sensitive and accurate quantitative assays for enzyme(s) protein in rabbit tissues. Radioimmunoassay techniques for native enzymes will then be systematically applied in studies on metabolic regulation and on enzyme protein synthesis.